1. Field of the Invention
The present invention relates to a thin film transistor, an active matrix substrate, and an image pickup device.
2. Description of the Related Art
In recent years, the development of liquid crystal displays and organic EL (electroluminescence) displays has been advancing. These displays have drive systems classified into the passive matrix system or the active matrix system. In the active matrix system, a thin film transistor (TFT) is formed for every pixel to control the driving. Because of formation of the TFTs, the active matrix system has such an advantage that, although it has a complex structure, high image quality can easily be obtained, as compared with the passive matrix system.
Meanwhile, in wide fields including the medical, industrial and atomic energy fields, image pickup devices that take an image by irradiating electromagnetic waves such as X-rays are utilized. For example, the radiation image pickup device irradiates radiation to an object, detects the intensity of the radiation that has passed through the object, and thus obtains information about the inside of the object. These radiation image pickup devices are broadly classified into direct image pickup devices and indirect image pickup devices. The direct image pickup device employs a system of directly converting the radiation that has passed through the object into electric signals to be extracted to the outside, and the indirect image pickup device employs a system of causing the radiation that has passed through the object to be incident on a fluorescent material to convert it once into visible light, and converting the visible light into electric signals to be extracted to the outside.
In radiation image pickup devices for use as direct image pickup devices, generally, incident radiation (for example, X-rays) is directly converted into electric signals (charge) by an a-Se-based semiconductor film having sensitivity to the radiation. FIG. 6 schematically shows the basic constitution of a radiation sensor of a direct conversion type. The radiation sensor is provided with an active matrix substrate 100 having many collecting electrodes (not shown) formed thereon in a two dimensional matrix arrangement set in a radiation detection effective area SA and an electric circuit (not shown) for accumulating/reading out charges collected at respective collecting electrodes along with the incidence of the radiation, an a-Se-based semiconductor film 102 laminated on the collecting electrode-formed face side of the active matrix substrate 100, and a common electrode 104 for applying bias voltage laminated broadly in a planar shape on the front side of the a-Se-based semiconductor film 102.
To the common electrode 104, bias voltage is applied from a bias supply power source, and, in a state in which the bias voltage is applied, charges that are generated at the a-Se-based semiconductor film 102 along with the incidence of radiation to be detected and are collected at the respective collecting electrodes are extracted as a radiation detection signal for each of the collecting electrodes by the accumulating/reading out electric circuit composed of a capacitor, a switching element, electric wiring and the like.
For example, in order to construct a non-planar X-ray image pickup device, it has been proposed to use a flexible substrate, arrange three TFTs in one pixel, and form an active layer with an In—Ga—Zn—O-based amorphous oxide (see JP-A No. 2006-165530). It is described that the formation of the active layer with an In—Ga—Zn—O-based amorphous oxide leads to a carrier concentration of less than 1018/cm3 to attain a normally-off operation.
In addition, as a TFT for use in organic EL displays of the active matrix system, a TFT having a so-called double-gate structure, in which an active layer is formed with an In—Ga—Zn—O-based amorphous oxide and a gate electrode is arranged on both sides of the active layer, is disclosed (see “Nikkei Electronics,” Nikkei Business Publications, p 104, May 5, 2008). It is reported that, by forming an IGZO-based TFT into a double-gate structure and controlling both gate electrodes with the same voltage, an apparent electron field-effect mobility increases significantly as compared with a case of one gate electrode, and that, when the gate voltage is 0 V, off current decreases as compared with general double-gate structure TFTs.
When an amorphous oxide semiconductor is used as an active layer of TFTs, although there is such an advantage as high in-plane uniformity of threshold voltages, it is difficult to assure drive stability of the threshold value. When it is attempted to suppress the variation of the threshold value due to the driving, the carrier concentration in the active layer becomes comparatively high, and the TFT is apt to exhibit a normally-on operation. The normally-on operation causes such a problem as the necessity for an additional power source.
It is practically necessary for the normally-off operation that the carrier concentration of the active layer is less than 1016 cm−3. In this case, however, there is such problem that the threshold value is apt to vary.
Further, there is also such a problem that an insulator exists on the back-channel side of a TFT, and that electrification by static charge changes the threshold value of the TFT. When used, particularly, as a direct conversion type X-ray image pickup device, charges generated by X-rays are apt to electrify the back-channel to easily vary the threshold value.
On the other hand, when arranging two gate electrodes based on the double-gate structure and controlling them by the same electric potential, it is necessary to provide a contact hole in the interlayer insulating film and to connect electrically the two gate electrodes. This makes the manufacturing process complex and thus raises the manufacturing cost significantly. Further, in the case of a double-gate structure in which two gate electrodes are controlled by the same electric potential, there is such a problem that power consumption increases as compared with the case of the single-gate structure.